- I think you presented the quote fairly; stating that you thought it was from Tolkien's letters, and not saying outright that it was from Tolkien himself. I admit I found it unlikely; I don't think JRR would use the phrase ' they brought down upon themselves a very world-shattering doom'.

And Voronwe was right in pointing out that it most prob'ly comes from an online forum; a quick google of the first sentence gives us this webpage -

http://forum.barrowdowns.com/archive/index.php?t-1311.html

One thing this does show is that it's easy to pluck stuff off the net; and that it's important to find out where it comes from, in order to ascertain what is Tolkien and what is not. I'm thinking of those web-pages with so-called 'Tolkien quotes' - few if any give citations, and all too often folk will repeat what they find there without question. At least that's not so in your case, and I'm grateful for that.